This invention has particular application to cartridge heaters, but its applicability is not confined to them.
In installing cartridge heater units, it is desirable that the cylindrical sheath of the cartridge be in intimate contact with the object to be heated. At the same time, it is not practical to have a press or interference fit between the wall of the hole in the object into which the cartridge is inserted and the cartridge, because the cartridge heater element will eventually fail, and it is necessary to remove the cartridge. Accordingly, it has been customary to drill and then ream the wall defining the hole into which the cartridge heater is to be inserted, to get as close a fit as possible, but yet to be able to remove the cartridge when it is to be replaced.
Refractory coatings containing a major proportion of boron nitride have been known as high temperature lubricants, anti-sticking agents and mold release agents, resistant to corrosion and to molton metals.
It has been found, that contrary to what might be thought to be a disadvantage of interposing a coating of refractory between a heating element and the object to be heated, the interposition of a refractory coating predominantly of boron nitride, under certain circumstances, provides a more efficient heat transfer and has certain other advantages as well. The increased efficiency alone provides a marked increase in the life expectancy of the heating elements.
One of the objects of the this invention is to provide metal sheathed elements more efficient than those know heretofore.
Another object is to provide a method of producing such elements that is simple and that facilitates mounting of the elements.
Other objects will become apparent to those skilled in the art in the light of the following disclosure.